Adding M365 Shared Mailbox in New Outlook
If you add the permissions in Exchange, the shared mailbox should appear in the new outlook under the shared with me folder after a few minutes. You can expand it to see if this is the case.
->If it is still not appearing, you can try to add it with the following steps and check whether the shared mailbox displays in your Folder list under this folder.
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For more information, please also see the article Working with Generic and Shared Mailboxes.
Adding a shared mailbox to new Outlook
Open Outlook.
Right-click on the account name or shared with me folder in the left navigation pane, and then choose Add shared folder or mailbox.Â
In the Add shared folder or mailbox.  dialog box, type the name or email address of the shared mailbox, and then click Add.
You can view the shared mailbox under “shared with me“ option.
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Notes:
The mailbox folders and contents can take an hour or two to appear if your permission to access the shared mailbox was added on the same day when you add the shared mailbox to Outlook.
Sent messages from this shared mailbox will be saved in the Shared Mailboxes' Sent folder
To learn more, please refer to the article Open and use a shared mailbox in Outlook - Microsoft Support
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Need help?
Contact the IST Service Desk online or 519-888-4567 ext. 44357.